In systems for the magnetic recording of digital information, it has previously been proposed to convert the input data into special longer code groups and to indicate the presence of a "1" by a magnetic transition from one state to another. An article which discloses such a system and its advantages is entitled "A New Look-Ahead Code for Increased Data Density" by George V. Jacoby, presented at INTERMAG, June, 1977.
In the Jacoby article, three data bits are converted into six code bits, with each binary "1" being spaced apart by at least two binary "0's", and this restriction is carried over even to interfaces between adjacent code groups through certain logical algorithms. However, the nature of the system proposed in the above-mentioned article is such that transitions from one magnetic state to the other need occur only every second code group, or once every 12 code bits and that data patterns may occur when the spacing is repeated indefinitely. This requires a very tightly controlled phase-locked oscillator that is capable of maintaining accurate clocking with a sustained pattern of one flux transition every 12 clock periods. In addition, the selection of groups of three data bits to produce the six bit code groups which are formed is generally incompatible with most data processing circuits which utilize and process digital information in eight bit "bytes", or in four bit half-bytes, where two half-bytes form a byte. This incompatibility results in the need for additional buffering circuitry as the information is transferred from the data processing circuit to magnetic storage, and additional buffering as the groups of three bits are received from magnetic storage and supplied back to the data processing circuitry which operates with eight bit bytes or four bit half-bytes.
Accordingly, one important object of the present invention is to develop a high density magnetic recording system which is efficient and inexpensive with data processing systems which operate on eight bit bytes or four bit half bytes.
Another object of the present invention is to increase the minimum sustained frequency of magnetic transitions in the encoded data, without sacrificing density of recording, to thereby substantially reduce the constraints or requirements on the phase lock oscillator which generates the timing or clocking signals for the system.